The present invention relates to an apparatus for burying a pipeline in the bottom of a body of water, and more particularly relates to a jet sled for entrenching a pipeline in a seabed consisting of unconsolidated material.
It has become common practice to entrench submerged marine pipelines beneath the surface of the ocean floor. This has been found necessary to prevent damage to the pipeline, as was experienced when pipelines were laid and left to rest uncovered upon the ocean floor.
Conventional apparatus for entrenching a submerged pipeline has included a submerged jet sled. Typical jet sleds include a frame structure for straddling the pipeline with port and starboard pontoon runners to support the structure. A conventional jet sled is provided with port and starboard high pressure jetting nozzles which, upon being supplied with fluid, such as air or water, dislodge soil from the seabed to form a trench for the pipeline. The jet sled is pulled along the ocean floor by a surface vessel which also supplies the necessary high pressure fluid for the jetting nozzles.
The conventional jet sled works quite well in seabed formations consisting of cohesive materials. However, when a pipeline is desired to be buried in a seabed consisting of unconsolidated materials such as sand, clay or other sedimentary materials, difficulty has been experienced in keeping the trench from filling up before the pipeline can come to rest at the proper depth within the trench. Problems of this type are especially acute when a pipeline is desired to be buried in a seabed consisting almost entirely of sand.
One approach to burying a pipeline in the bottom of a body of water consisting of an unconsolidated material is that of fluidizing the sea bottom proximate the pipeline, thereby permitting the pipe to sink by its own weight into the fluidized sea bottom. This approach to pipeline burying does not involve the formation of a trench in front of the pipeline, but rather the entire burying operation is accomplished by a series of fluidizing sleds spaced at intervals indisposed underneath the pipeline to locally fluidize the bottom material. This approach to pipeline burying is illustrated by De Geeter, U.S. Pat. No. 3,659,425.
Another approach to the problem of burying pipelines in unconsolidated seabed material involves the disposition around the underside of the pipeline of a number of tubes having fluidization openings or jets provided along their length. The tubes extend completely around the underside of the pipeline and are held in place by a number of spaced apart yokes. Fluid is released under high pressure through the openings into the bottom adjacent to the pipeline in order to fluidize the unconsolidated material. The weight of the pipeline will cause the pipeline to sink into the fluidized bottom material until it has reached the desired depth. This approach to pipeline burying is disclosed in Van Steveninck, U.S. Pat. No. 3,702,540.
Another effort to overcome the problem of pipeline burying in unconsolidated seabeds is that of providing fluidization pipes affixed to and running along the pipeline to fluidize the seabed around the pipeline, causing the pipeline, together with the fluidization pipes, to sink into the fluidized seabed. This approach is that of Van Steveninck, U.S. Pat. No. 3,695,049.
The foregoing approaches to burying pipelines in seabeds comprising unconsolidated material have involved the fluidization of the seabed material adjacent to pipeline to permit the pipeline to sink by its own weight into the fluidized bottom material. All of the foregoing techniques dispense with the formation of a trench ahead of the pipeline and utilize local fluidization.
Another characteristic of the foregoing described techniques for burying a pipeline in a seabed consisting of unconsolidated material is that all require that a great amount of fluid be released from the fluidization nozzles in order to maintain the seabed material in a completely fluidized state as is necessary for satisfactory pipeline burial.